Insulin: The Protein That Controls Blood Sugar

Insulin is a small protein with an enormous story. It was the first protein hormone to be purified for medical use, the first protein ever to have its amino acid sequence determined, one of the first protein structures solved by X-ray crystallography, and the first medicine made by genetically engineered bacteria. Every day, millions of people with diabetes depend on it. Its chemistry explains how it works and how scientists learned to improve it.

Identity

Property	Human insulin
Type	Peptide hormone
Amino acids	51, in two chains
A chain	21 amino acids
B chain	30 amino acids
Disulfide bridges	3 (2 between chains, 1 within the A chain)
Molar mass	about 5,808 g mol⁻¹
Made by	β cells in the islets of Langerhans (pancreas)

Structure

Insulin has two polypeptide chains, A and B, held together by disulfide bridges between cysteine side chains (see amino acids):

• two interchain disulfide bridges link the A chain to the B chain;
• one intrachain disulfide bridge links two cysteines within the A chain.

The chains fold into a compact shape, with α-helices in both chains, stabilised by a small hydrophobic core (see protein structure levels).

In the β cells, and in many pharmaceutical preparations, insulin molecules pair up into dimers, and three dimers assemble around two zinc ions into hexamers. The hexamer is a compact storage form. After injection, hexamers must slowly break apart into single molecules (monomers) before they can be absorbed into the blood and act, which affects how quickly an insulin preparation works.

How the body makes insulin

Insulin isn’t made as two separate chains. It’s made as one longer chain that is then cut:

1. Preproinsulin is made by ribosomes. Its “signal peptide” directs it into the cell’s secretory pathway and is then removed.
2. The remaining proinsulin is a single chain of 86 amino acids. It folds, and the three disulfide bridges form in the correct positions.
3. Enzymes cut out a middle section, the C-peptide (about 31 amino acids), leaving the A and B chains held together by the disulfide bridges.
4. Insulin and C-peptide are stored together in granules and released together.

Making insulin this way solves a chemical problem: folding a single chain puts the cysteines in the right places to pair correctly, which is much harder when two separate chains must find each other. Doctors measure C-peptide in blood as a marker of how much insulin a person’s own pancreas is making, since injected insulin contains no C-peptide.

What insulin does

After a meal, blood glucose rises. β cells sense this and release insulin. Insulin then acts on its target cells:

1. Binding: insulin binds the insulin receptor on the cell surface, a protein that spans the membrane.
2. Activation: binding changes the receptor’s shape and activates its built-in tyrosine kinase, an enzyme that attaches phosphate groups (from ATP) to tyrosine side chains, starting with the receptor itself.
3. Signalling cascade: the phosphorylated receptor activates a chain of other proteins inside the cell (see hormones as chemical messengers).
4. Effects:
   • in muscle and fat cells, glucose transporter proteins (GLUT4) move from inside the cell to the membrane, so glucose enters much faster;
   • in the liver and muscle, glucose is converted into glycogen for storage (see starch vs glycogen vs cellulose);
   • in fat tissue, fat storage increases and fat breakdown decreases;
   • the liver stops making new glucose;
   • protein synthesis increases.

The result is that blood glucose falls back to its normal range, about 4–7 mmol L⁻¹ (roughly 70–130 mg per decilitre). Between meals, the hormone glucagon does the opposite, releasing glucose from the liver. Together, the two keep blood glucose steady by negative feedback.

Diabetes

Diabetes mellitus is a group of conditions in which blood glucose stays too high:

• Type 1 diabetes: the immune system destroys the β cells, so the body makes little or no insulin. People with type 1 diabetes need insulin injections or pumps to survive.
• Type 2 diabetes: the body still makes insulin, but cells respond poorly to it (insulin resistance), and over time β cells may fail. It’s strongly linked to obesity and inactivity, and is treated with lifestyle changes, medicines and sometimes insulin.

Without insulin, cells can’t take up glucose efficiently even when blood glucose is very high. The body breaks down fat instead, producing acidic ketone bodies, and in type 1 diabetes this can lead to life-threatening ketoacidosis, a dangerous fall in blood pH (see the blood buffer system).

A history of firsts

• 1921–1922: In Toronto, Frederick Banting and Charles Best, working in John Macleod’s laboratory, extracted insulin from dog pancreases. Biochemist James Collip purified it enough for human use. In January 1922, 14-year-old Leonard Thompson, dying of diabetes, became the first person treated. Banting and Macleod received the Nobel Prize in 1923 and shared it with Best and Collip.
• 1955: Frederick Sanger determined the complete amino acid sequence of insulin, the first protein ever sequenced. It proved that each protein has a precise, defined sequence. He received the 1958 Nobel Prize in Chemistry (and a second in 1980, for DNA sequencing).
• 1969: Dorothy Hodgkin solved the three-dimensional structure of insulin by X-ray crystallography, a project that took her more than 30 years.
• 1978–1982: Scientists inserted the human insulin gene into bacteria, which then produced human insulin. In 1982, it became the first medicine made by recombinant DNA technology to be approved. Before this, insulin came from pig and cow pancreases; pig insulin differs from human insulin by just one amino acid, cow insulin by three.

Insulin analogues: redesigning a protein

Using genetic engineering, chemists can change insulin’s amino acid sequence to alter how fast it acts:

• Rapid-acting analogues: in insulin lispro, two amino acids near the end of the B chain (proline and lysine at positions 28 and 29) are swapped. This small change weakens the contacts that hold insulin dimers together, so the hexamers break apart quickly after injection and the insulin acts within about 15 minutes, suited to mealtimes.
• Long-acting analogues: insulin glargine has changes that make it soluble at the slightly acidic pH of the injection but insoluble at body pH. It forms a tiny deposit under the skin that dissolves slowly, releasing insulin steadily for about a day. Other long-acting insulins have a fatty acid chain attached so they bind to albumin in the blood.

These are clever examples of using acid–base chemistry and protein structure to design better medicines.

Why insulin can’t be swallowed

Insulin is a protein, so in the stomach and intestine it’s denatured by acid and hydrolysed by digestive enzymes into amino acids, like any other dietary protein (see protein denaturation). It’s also too large to be absorbed intact. That’s why it’s injected under the skin. Researchers are developing oral and inhaled forms, but they face exactly these chemical obstacles.

Key takeaways

• Insulin is a 51-amino-acid protein with A (21) and B (30) chains joined by disulfide bridges.
• It’s made as proinsulin, which folds and is cut to release insulin and C-peptide; it’s stored as zinc-containing hexamers.
• Insulin binds a tyrosine kinase receptor, triggering glucose uptake (via GLUT4), glycogen storage and fat storage, lowering blood glucose.
• Lack of insulin or response to it causes diabetes.
• Insulin was the first protein sequenced (Sanger) and the first recombinant medicine; engineered analogues change how fast it acts.